Optical fibers are being used in a variety of biosensor applications. For example, as discussed in U.S. Pat. No. 5,494,798, a fused biconical fiber optic fiber coupler may be used without cladding to exploit the evanescent field present immediately outside the fiber/air interface in its “waist” region. If an antibody is bound to the exposed surface of the bare waist region of the fiber optic coupler, the evanescent field envelopes the molecule. But since there is little or no absorption or other phenomena to alter the amount of the light carried by the fiber, no attenuation or detectable characteristics are developed.
However, when the antibody's target antigen binds to the antibody, the localized changes in refractive index in the evanescent field cause characteristic changes in the ratio output of the fiber optic coupler.
Whereas previous fiber-optic evanescent-wave sensors utilized multi-mode fibers and are primarily based on fluorescence, the '798 patent improved on the technique by employing a pair of single-mode optical fibers in a coupler arrangement measuring changes in refractive index. Light is introduced into one of the fibers to produce an evanescent region surrounding the coupling area, and the magnitude of light emitted from the pair of fibers is compared for detection purposes.
Light from a laser diode is inserted into a first leg of the fiber optic coupler, and exits from the same fiber, forming an input channel. The second fiber of the coupler provides an output channel for light from the first leg. A first photo diode detector is connected to the input channel and a second photo diode detector is connected to the output channel. Each detector feeds its own transimpedance amplifier, the outputs of which are applied to A/D converters providing digital electrical signals to an instrumentation board and attached personal computer which outputs results to a printer and monitor.
The finished probe includes the fiber optic coupler and attached antibodies, which yields a baseline ratio for the sensor. The finished probe is then exposed to a material of interest, and the ratio of the light through the two sides of the coupler changes as a function of the way in which the target attaches. That is, the localized index of refraction at the coupling region and the determination of the ratio is a function of the binding of the molecular target to the bound receptor in the coupler region.
Though versatile for some applications, the apparatus just described is limited in terms of applicable optical characteristics as well as amenability to large-scale production. Difficulties in construction lead to poor, irreproducible operational characteristics. This in turn minimizes the applicability for using polarization, interference and other potentially useful optical phenomena in favor of a strict magnitude comparison. Thus, the need remains for a more versatile arrangement utilizing evanescent field detection which affords greater sensitivity while being conducive to larger-scale production at low cost.